Add Python 3.14 support, drop Python 3.9

[mortenwh]

Summary

Add Python 3.14 support and drop Python 3.9 (EOL October 2025).

Changes required

pyproject.toml

• Remove "Programming Language :: Python :: 3.9" classifier
• Add "Programming Language :: Python :: 3.14" classifier
• Update requires-python = ">=3.10"

.github/workflows/tests.yml

• Update test matrix from ["3.9", "3.10", "3.11", "3.12", "3.13"] to ["3.10", "3.11", "3.12", "3.13", "3.14"]

Dependency compatibility

All key dependencies should be verified to support Python 3.14 before the PR is merged:

• lxml (currently 6.1.1)
• netCDF4 (currently 1.7.4)
• shapely
• pandas
• parmap
• metvocab (pinned to v1.2.0 via git)

Note: Python 3.14 is expected to reach stable release in October 2026. CI tests may need to use allow-failure or pre-release builds until then.

References

• Python 3.9 EOL: https://devguide.python.org/versions/
• Similar previous work: Add Python 3.13 support #364 (Python 3.13 support), Fix Python 3.12 compatibility #362 (Python 3.12 compatibility)